Concept:
Fibre migration refers to the movement of fibres within the yarn cross-section during spinning and twisting processes. Fibres do not remain fixed at a single radial position; instead, they continuously migrate between core and surface regions, improving yarn uniformity and structural stability.
Step 1: Interpret mathematical expression.
The given double integral represents distribution of fibre strain energy across:
• radial position \(r\)
• strain level \(\epsilon_f\)
This indicates variation of fibre behavior within yarn structure.
Step 2: Understand physical meaning.
Such integrals are used in modelling:
• fibre displacement
• internal stress distribution
• migration paths of fibres
Step 3: Link to concept.
Since the expression describes fibre movement and redistribution within yarn cross-section, it corresponds to fibre migration.
Step 4: Final conclusion.
\[
\boxed{\text{Fibre Migration}}
\]
Thus,
\[
\boxed{\text{Option (B) is correct.}}
\]